1.Field of the Invention
The present invention relates to a semiconductor device and an image display apparatus. More particularly, the present invention relates to a semiconductor device applied to display devices such as a liquid crystal display device and organic EL (Electro Luminance) display device, and an image display apparatus employing such a semiconductor device.
2.Description of the Background Art
A thin film transistor is used in a display device. As an example thereof, a thin film transistor of a GOLD (Gate Overlapped Lightly Doped Drain) structure disclosed in Document 1 (Japanese Patent Laying-Open No. 2002-076351) will be described hereinafter. An n channel type thin film transistor of a GOLD structure has a source region, a drain region, a channel region, a GOLD region, a gate insulation film, a gate electrode, and the like, formed on a glass substrate.
The GOLD region is formed at a region between the channel region and the drain region, particularly at a region located right under the gate electrode. The GOLD region is formed overlapping with the gate electrode in plane. The GOLD region is set to have a higher impurity concentration than the channel region and a lower impurity concentration than the drain region.
The operation of an n channel type thin film transistor, for example, of the GOLD structure will be described here. A channel is formed at the channel region when a predetermined positive voltage is applied to the gate, whereby the resistance across the source region and the drain region is reduced to allow a current flow across the source region and the drain region. When a negative voltage is applied to the gate, the resistance across the source region and the drain region is increased since a channel is not formed at the channel region. Therefore, no current substantially flows across the source region and the drain region. Only a small leakage current will flow.
This leakage current is caused by the recoupling at the junction between holes formed at the channel and many electrons located at the source and drain regions. Since the probability of recoupling is increased when the electric field at the junction becomes higher, leakage current will be increased.
In a display device, the voltage applied to the liquid crystal must be maintained for the duration of one frame until the screen is rewritten. If leakage current at the pixel thin film transistor employed for retaining the voltage is great, the voltage applied to the liquid crystal will be decreased over time to degrade the display property. It is therefore necessary to minimize the leakage current in a pixel thin film transistor.
A thin film transistor of an LDD (Lightly Doped Drain) structure disclosed in Document 2 (Japanese Patent Laying-Open No. 2001-345448) will be described hereinafter as another example of a thin film transistor employed in a display device. An n channel type thin film transistor of the LDD structure has a source region, a drain region, a channel region, an LDD region, a gate insulation film, a gate electrode, and the like, formed on a glass substrate. The LDD region is formed at a region between the channel region and the drain region. The LDD region is set to have a higher impurity concentration than the channel region and a lower impurity concentration than the drain region.
In a thin film transistor of an LDD structure, application of a negative voltage as the gate voltage will cause an accumulation layer to be formed at the channel region. The electric field in the proximity of the source and drain is alleviated by the LDD region to allow the leakage current to be suppressed.
Conventional thin film transistors have problems set forth below. As mentioned above, a thin film transistor employed as a pixel thin transistor must have the leakage current suppressed to an extremely low level. In a thin film transistor of a GOLD structure that is one example of a conventional thin film transistor, application of a negative voltage as the gate voltage will result in formation of an accumulation layer at the GOLD region, whereby a high electric field is generated in the proximity of the source region and the drain region having an impurity concentration higher than that of the GOLD region. Therefore, leakage current could not be suppressed reliably.
Further, application of a voltage to the drain higher than that to the gate will generate a relatively large electric field at the junction region of the drain side. Electrons accelerated by this electric field induce impact ionization, whereby a pair of an electron and hole is generated. Impact ionization is repeated to increase the pairs of electrons and holes, causing increase in the drain current to result in avalanche breakdown. The drain voltage at this stage becomes the source-drain breakdown voltage.
Since the electric field in the proximity of the drain region is alleviated at the junction between the channel region and the GOLD region in the thin film transistor of a GOLD structure set forth above, impact ionization can be suppressed to a certain level. However, there was a problem that sufficient source-drain breakdown voltage could not be achieved by a GOLD region with the length in the direction of the channel length (GOLD length) under practical usage.
Similar problems are encountered in other examples of thin film transistors of an LDD structure. Specifically, when a channel is formed at the channel region in response to application of a positive voltage as the gate voltage, the resistance of the LDD region will be connected in series with respect to the channel resistance. Since the impurity concentration of the LDD region is lower than the impurity concentration of the source region and the drain region, the resistance at the LDD region will become higher to lead to the problem of lower ON current.
Since the electric field in the proximity of the drain region is alleviated at the junction between the channel region and the LDD region, impact ionization can be suppressed to a certain level. However, sufficient source-drain breakdown voltage as well as reliability with respect to AC stress could not be achieved by an LDD region with the length in the direction of the channel length (LDD length) under practical usage. Thus, conventional thin film transistors had the problem that sufficient source-drain breakdown voltage could not be achieved.